1. Field of the Invention
Embodiments of the present invention are directed to the field of photovoltaics (PV) technology that converts solar energy directly into electrical energy. The field of the invention is specifically directed to minimization of resistive loss of solar panels in order to achieve maximum solar energy conversion efficiency, extracting more electrical power from available solar irradiance.
2. Description of the Related Art
Practical solar cells have a number of limits including reflection, contact shadowing, series resistance, incomplete collection of photogenerated carriers, absorption in the inactive window layers, nonradiative recombination, etc. that can affect energy conversion efficiency. Among them, energy losses arising from ohmic resistance of the metal grid of the back contact of solar cells and of the electrical connections between the cells installed on a solar module may cause significant decrease in solar conversion efficiency and less power extraction than it should be. The primary sources of the electrical resistances that may contribute to energy losses include metallization, shadowing, as well as electrical wiring.
The resistance of the metallization is very critical to the conversion efficiency of a solar cell. Usually, the sheet resistivity ρs of the diffused surface layer dominates the resistivity losses for current flowing in the surface of the cell. The sheet loss resulted from the fingers of a metal grid on a cell surface is given byPs=S2ρsJo2,  (1)where S is the spacing between parallel fingers that form a metal grid on the surface of a solar cell, Jo is the current density. This is valid generally when the spacing between the parallel fingers of a metal grid on a solar cell is large comparing to their width.
The resistance of the metal grids that are in direct contact with the surfaces of solar cells may affect conversion efficiency drastically. The resistive loss from a rectangular metal finger of resistivity ρm is proportional to its length to the power of three as given by (H. B. Serreze, Proc. 13th IEEE Photovoltaic Spec. Conf. (IEEE, New York, 1978), p. 609):Pm=WL3SρmJo2/3D,  (2)Where W is the width of a rectangular cell, L is the finger length on the cell, and D is the width of the finger.
The shadowing loss due to the metal grid on the surface of a solar cell is, in addition to the geometrical parameters of the metal fingers on a cell, proportional to the maximum power voltagePs=WLJoVmpD/S.  (3)
The optimum finger width can be derived from minimizing the sum of Pm and Ps,Do=LSJo(ρm/3Vmp)1/2,  (4)and for which the total power loss isPT=2WL2(ρmJo3Vmp/3)1/2.  (5)
These extrinsic sources of energy loss, in principle, can be minimized, if not eliminated. The back contact structure developed in the past that has put both polarities of contacts on the backside of a solar cell (FIG. 1) gets rid of the front metal grid therefore completely eliminates the shadowing loss. It also reduces the series resistance of metal grid because the contacts can be very broad covering almost entire back surface. These structures are generally called back-contact solar cells are disclosed, e.g., in U.S. Pat. No. 4,478,879 and Van Kerschaver et al. (Back-contact Solar Cells: A Review. Prog. Photovolt: Res. Appl. 2006, 14:107-123).
Since the resistance of metal fingers is proportional to L/D, there is still a relatively large resistance that may cause considerably energy loss because the connection bus for both electrical polarities of a back-contact solar cell is at the opposite edges. Energy loss is inevitable when current flows through those narrow and thin fingers across almost entire length of the cell. It is obvious that shortening the length of metal fingers on the surface of solar cells or increasing the thickness of the fingers may further reduce the energy loss due to series resistance can definitely reduce such a loss. However, it is rather difficult to realize all these on the cell level due to various constrains associated with materials properties and related to production process and manufacturing cost issues.